The invention relates generally to current sensing systems. In particular, the invention relates to optically interrogated systems.
Measurement of currents flowing in high-voltage environments is highly desirable, especially in power transmission and distribution systems. Transmission systems react dynamically to changes in active and reactive power. For power transmission to be economical and the risk of power system failure to be low, reactive compensation systems are desirable, particularly systems capable of simultaneously monitoring current flow at several points on a grid.
High-voltage current transformers (CTs) are traditionally used in the utility industry to measure currents flowing on transmission lines at voltages up to 735 kV. Use of high-voltage CTs is very costly, ranging into the hundreds of thousands of dollars, because of the cost of large, oil-filled insulating columns that provide the mechanical support for a large current transformer and ensure sufficient dielectric insulation from measurement point to ground.
As an alternative to high-voltage current transformers, optical current sensors are sometimes used. Optical current sensors typically rely upon the Faraday effect, whereby the magnetic field created by the alternating current alters the polarization of light flowing in fiber near the conductor. The method for extracting this information from fiber is very intricate and costly. In one example complex, active power supplies are located at line potential. These supplies derive electrical power from the transmission line or a ground-based laser and provide power to electronics that actively sample, multiplex, and transmit optical digital signals over fiber to ground-potential equipment.
There is a need therefore for low cost high-voltage current metering and instrumentation. There is a particular need for a current measuring system that allows for multiplexing, which can be economically scaled and used in applications where instrumentation of multiple channels is required. Additionally, a completely passive current sensor, eliminating any need for auxiliary power circuits at the high-voltage level, is highly desirable in high-voltage equipment, where a sensor failure can require costly outages to allow for repair.